Successful endosseous implants date from about 1968, at which time a biocompatible metal blade was fitted into a prepared elongated receptor site. The blade itself was perforated or vented to allow bone and blood vessels to reunite readily. A projecting metal head, either unitary with or detachable from the blade, provided an anchor for attachment of a fixed bridge. Another endosseous metal implant design is the basket type having a projecting metal head. This implant is used specifically for partial support of a fixed bridge. Neither the blade nor the basket implant is designed or adapted for use as an anchor for overdentures or for use as a free-standing single tooth replacement to support a single crown. Solid root-shaped ceramic implants such as sold by Miter Co. of Worthington, Ohio, are used for support of a single tooth replacement and for partial support of a fixed bridge, but are not adapted for use as an anchor for overdentures. Ceramic implants are stiff, brittle and bulky. They are not susceptible to specific case modifications.
The screw-type implants have been attempted for overdenture retention. The screw-type implant, such as sold by Zest Anchors, Inc. of San Diego, Calif., and Institut Shausman AG of Waldenburg, Switzerland, utilizes a solid self-tapping screw.
A relatively small percentage of such sold screw-type implants have long term success. Commonly, failure occurs due to inadequate reattachment of bone tissue to the screw. Even in cases that appear to provide good connection, bone deterioration at the neck of the implant, as well as bone resorption at the threads, is often radiographically detected.
The basket type implant utilizes an inverted cylindrical cage or "basket" that fits about a cylindrical bone core and into an annular recess that is formed by an incomplete cut of a trephine drill. Perforations or vents in the basket or cage allow bone and blood vessels to reunite. A good mechanical fixation is accomplished, but only after a fairly lengthy healing period. Clearly, the screw-type implant is advantageous from the standpoint of immediate structural connection.
The primary object of the present invention is to provide an implant system (1) that has a higher success rate as compared to the blade, screw and basket types of metal implants and compared to ceramic implants; (2) that is versatile enough to be used for the retention of an overdenture, the support of a fixed bridge, or the support of a crown for a single tooth replacement; (3) that isolates the endosseous portion from the oral cavity during the healing state; and (4) that allows the actual head portion to become a part of the working laboratory model during construction of the fixed prosthesis capable of modification without annoyance to the patient prior to completion of the final prosthesis.
Another important object of this invention is to provide an implant system that utilizes a screw anchor so constructed that it can be precisely trimmed (1) to fit the specific jawbone recess; (2) to locate the screw threads at the required minimum distance beneath the bone crest; and (3) to locate the top of the anchor just slightly above the bone crest.
Another object of this invention is to provide a two-part endosseous implant that includes as one part, a semi-rigid (or semi-flexible) pillar supported by the screw anchor adjacent the level of the bone crest to maximize stability relative to lateral forces.
Another object of this invention is to provide a plastic pillar of this character that has a flexural modulus of elasticity close to that of bone tissue itself and that can be trimmed, as need be, to change flexural characteristics closely to approximate the slight mobility found in natural teeth. Occlusal stresses are thus evenly transmitted to the bone. Moreover, by providing a slight yielding or shock absorbing function, harmful stresses upon application of lateral occlusal forces are reduced. Avoidance of localized stress promotes an intimate and lasting contact between the bone and the metal screw anchor, not achieved by other implant systems.
Still another object of this invention is to provide a special trephine drill to facilitate the precise preparation of the jawbone recess.